1. Field of the Invention
The present invention relates to a driving circuit for a vibration wave motor.
2. Related Background Art
The present applicant already disclosed, in the Japanese Laid-open Patent Sho 63-23572, a driving circuit for a vibration wave motor. FIG. 2 illustrates the device disclosed in said Japanese Laid-open Patent Sho 63-23572. wherein a stator (vibration member) 1 of the vibration wave motor is provided with drive electrodes 1-1, 1-2, a resonance state detecting electrode 1-3, and a common electrode 1-4. A phase comparator 2, a low-pass filter 3, a voltage-controlled oscillator 4 and a 1/32 frequency divider 5 constitute a phase-locked loop (PLL) for generating a frequency, from the oscillator 4, of 32 times compared to that of the output of an oscillator 19 to be explained later. There are provided shift registers 6, 7 for phase shifting, in which the shift register 6 is used for generating a signal of a phase difference of 90.degree. to be supplied to the drive electrodes of the vibration wave motor, while the shift register 7 is used for generating a phase difference necessary for causing the drive frequency to follow the resonance frequency of the motor. As said shift registers 6, 7 receive, at the clock input terminals thereof, a frequency of 32 times of the driving frequency of the vibration wave motor, there is conducted a phase shift of 360.degree./32=11.25.degree. per stage of said shift registers. AND gates 9, 10 transmit the signals necessary for driving the vibration wave motor, only when an H-level signal is supplied to a START/STOP signal input terminal. The outputs of said AND gates 9, 10 are respectively connected to amplifiers 11, 12, of which outputs are respectively connected to the drive electrodes 1-1, 1-2 of the motor through coils 13, 14. Said amplifiers 11, 12 serve to amplify the output signals of the AND gates 9, 10 to a voltage necessary for driving the vibration wave motor, while the coils 13, 14 constitute serial resonance circuits in combination with the stator electrodes of the motor, for amplifying the drive signal and selecting a single frequency. Level comparators 15, 16 respectively are provided for detecting the phase of the signal wave of the drive electrodes 1-1, and for detecting the phase of the signal wave from the resonance state detecting electrode 1-3. The output of said level comparator 15 is supplied to a data input terminal of the shift register 7, of which output terminal is connected to the input terminal of a phase comparator 17. Also the output of the level comparator 16 is supplied to the input terminal of a phase comparator 17, of which output is supplied through a low-pass filter 18 to a voltage-controlled oscillator 19 for generating the drive signal for the vibration wave motor. The above-mentioned elements 17, 18, 19 constitute a phase locked loop (PLL). The vibration wave motor is included in said loop and is controlled following the resonance frequency thereof. In the above-explained structure, the output of the oscillator 19 is supplied to the electrodes 1-1 and also to an input end D of the shift register 6. Since said register 6 receives, at a clock input terminal thereof, the output of the oscillator 4 having a frequency of 32 times of that of the oscillator 19, an 8th-stage output OUT of said register 6 is shifted by 90.degree. with respect to the output of the oscillator 19 and is supplied to the electrodes 1-2. Thus the electrodes 1-1, 1-2 receive periodical voltages with a mutual phase difference of 90.degree. to generate a travelling vibration wave in said stator 1, thereby driving a movable member 1A maintained in contact with said stator. As described in the cited Japanese Laid-open Patent, electromechanical energy conversion elements such as piezoelectric elements or electrostrictive elements are provided on the electrodes 1-1, 1-2 formed on the stator, and generate said travelling vibration wave in response to the above-mentioned periodical voltages of different phases supplied to said electrodes.
The signal to the electrodes 1-1 is also supplied to an input terminal D of the register 7, which also receives the output clock signal of said oscillator 4, so that said register 7 releases, from an output terminal OUT thereof, a signal shifted by a certain amount in phase with respect to the signal supplied to the electrodes 1-1. In a predetermined position of the stator 1 there is provided a piezoelectric or electrostrictive element for monitoring the vibration state of said stator, and the output of said element is detected by an electrode 1-3. The vibration wave motor has a property that, in the resonance state, the drive signal supplied to the electrodes 1-1 and the output signal monitored by the electrode 1-3 show a certain phase difference. Thus, if the amount of phase shift in said register 7 is selected equal to said certain phase difference, the phase difference between the input signals to the comparator 17 becomes zero at the resonance state but becomes larger as the drive state is displaced from the resonance state. Therefore said phase difference is detected by a phase-locked loop composed of the elements 17, 18, 19, and the drive control is so conducted as to realize the resonance state.
More specifically, if the output frequency of the voltage-controlled oscillator 19 is higher than the resonance frequency of the vibration wave motor, the phase of the output signal from the electrode 1-3 is delayed in comparison with that in the drive with the resonance frequency. This delay is detected by the level comparators 15, 16, shift register 7 and phase comparator 17 whereby the phase comparator 17 releases an L-level output signal, which is supplied through the low-pass filter to the voltage-controlled oscillator 19 to reduce the output frequency thereof. On the other hand, if the output frequency of the voltage-controlled oscillator 19 is lower than the resonance frequency, the electrode 1-3 releases an output signal with advanced phase, which causes the phase comparator 17 to release an H-level output signal, whereby the input voltage of the voltage controlled oscillator 19 is elevated to increase the output frequency thereof. In such conventional device, therefore, the control is so conducted that the motor is always driven with the resonance frequency.
However such conventional device is associated with drawbacks that the start and stop of the motor are abrupt and involve shocks because the motor is immediately driven in the resonance state in response to an H-level signal given to the START/STOP terminal, and that the regulation of revolution of the motor is difficult to achieve.
In order to eliminate these drawbacks, the present applicant already proposed, in the Japanese Patent Application Sho 62-41862, a device shown in FIG. 3, in which same components as those in FIG. 2 are represented by same numbers and will not be explained further. A programable oscillator 8, with an oscillation frequency determined by a signal from a CPU 24, releases the drive signal for the vibration wave motor, corresponding to the output signal of the voltage controlled oscillator 19 shown in FIG. 2. The AND gates 9, 10 are connected, at an input terminal each, to an output port of the CPU and control the start and stop of the vibration wave motor according to the output signal of the CPU. The outputs of the shift register and the level comparator 16 are supplied to a phase comparator 20 which is so constructed as to release an H-level output signal in a period from the upshift of the output signal of the shift register 7 to the upshift of the output signal of the level comparator 16, and said output signal is supplied to an enable input terminal of a counter 21. A clock input terminal of said counter 21 is connected to the output terminal of the voltage-controlled oscillator 4 for generating a frequency of 32 times. Consequently a count of the counter 21 corresponds to a phase difference of 11.25.degree.. Parallel outputs of the counter 21 are supplied to data input terminals of a magnitude comparator 22. A data latch 23 latches an output value from the CPU 24 and sends outputs to other data input terminals of the magnitude comparator 22, which compares two input data and, when the data released from the counter 21 become smaller, releases an H-level signal from an output terminal connected to the CPU 24.
In the above-explained structure, when the vibration wave motor is driven by an H-level signal from the CPU 24 to the AND gates 9, 10, a phase difference corresponding to the frequency set in the oscillator 8 is supplied to the input terminal of the phase comparator 20. If the frequency set in the oscillator 8 is higher than the resonance frequency, the phase of the output signal of the level comparator 16 is delayed in comparison with the resonance state, so that the H-level state of the output signal of the phase comparator 20 becomes longer. Thus the counter 21 is enabled for a longer period and releases larger output data. The CPU 24 sets certain data in the data latch 23 in advance, in such a manner that the output data of the counter 21 become larger than the data set in said data latch 23 when the drive frequency is higher than the resonance frequency. Consequently the magnitude comparator 22 releases an L-level output signal when the drive frequency is higher than the resonance frequency. The CPU 24 gradually reduces the frequency set in the oscillator 8, according to a control program, during the L-level output from the magnitude comparator 22 for the purpose of speed control of the motor, thereby gradually bringing the motor toward the resonance drive state (high speed rotation state). In the course of variation of frequency explained above, there may be encountered inconveniences such as an abrupt decrease in the revolution of the motor if a frequency lower than the resonance frequency is selected.
The vibration wave motor has a property, as shown in FIG. 4A, of attaining a high speed at the resonance frequency fr but abruptly stopping if the frequency is reduced beyond said resonance frequency. For this reason, in the circuit shown in FIG. 3, the comparator 22 sends an H-level signal to the CPU 24 as shown in FIG. 4B when the count of the counter 21 becomes smaller than a predetermined value of the latch 23 which is selected slightly larger than the count of the counter 21 in the resonance state, whereby the drive frequency is set higher, by a predetermined amount, than the current frequency, and is therefore prevented from becoming lower than the resonance frequency. This circuit can therefore gradually bring the motor to the high speed rotation state, can also arbitrarily select the rotation speed through the selection of frequency, and can still prevent the stoppage of motor resulting from a reduction of frequency beyond the resonance frequency.
In practice, however, the phase relationship between the drive signal of the vibration wave motor and the signal from the detection electrode is not always detectable in stable manner but varies slightly depending on the rotation angle of the motor, and the signals involve high frequency noises. For this reason, if the vibration wave motor is driven at a high speed in a state close to the resonance frequency, the CPU 24 sets the frequency so as to obtain an L-level output signal from the comparator 22 and frequently changes the frequency set in the oscillator 8 due to the fluctuation in phase and the noises mentioned above. Such operation has resulted in mechanical vibrations and noise sounds in the vibration wave motor.
More specifically, when the comparator releases an H-level signal, the current drive frequency is reset to a somewhat higher frequency as explained above. As a result the comparator 22 releases an L-level signal to again enable a reduction of the frequency, whereby the drive frequency is somewhat reduced. These processes are thereafter repeated, so that the drive frequency always fluctuates by a small predetermined amount.
This drawback can be prevented by simply prohibiting the reduction of the drive frequency to a lower frequency when an H-level signal is released from the comparator 22. However such simple prohibition of change in the frequency may result in an abrupt stoppage of the motor, in case of a change (increase) of the resonance frequency resulting for example from the above-mentioned fluctuation in phase, as the drive frequency which is higher than the resonance frequency and which is subjected to the prohibition of frequency change, may become lower than the thus varied resonance frequency.